1. Field of the Invention
The present invention concerns a method of producing an RBa2Cu.sub.3 O.sub.y microcrystalline powder structure in which R denotes a lanthanide chosen from Y, La, Nd, Sm, Eu, Gd, Tb, Ho, Er, Tm, Yb and Lu and the value of Y is between 6.9 and 7.
2. Description of the Prior Art
Expansion of the field of application of superconductive ceramics is currently limited by our inability to produce materials with a high critical current density with reproducible electrical properties. According to the article "High Critical Current Density in Neutron Irradiated Bulk YBa.sub.2 Cu.sub.3 O.sub.7 " published in Modern Physics Letters B, 4 (11), 703 (1990), values of a few 10.sup.5 Acm.sup.-2 have recently been achieved for textured and then irradiated materials although the critical transport current for polycrystalline ceramics remains limited to 1.times.10.sup.3 Acm.sup.-2. This value is very far removed from the expected value.
Intergranular phenomena and the microstructure have a decisive role. It seems from the publication "Grain size dependence of the critical current density in YBa.sub.2 Cu.sub.3 O.sub.x " Appl. Phys. Lett. 55(26) 2781 (1989) that the critical current density is optimal for ceramics with a particle size between 1 and 2 .mu.m; this could be related to microcracking or to residual stresses as indicated in the publications "Microcracking in polycrystalline YBa.sub.2 Cu.sub.3 O.sub.7-x ", ICMC 90, Garmisch-Partenkirchen, Germany and "Demonstration of Grain Growth induced Microcracking and its Role in the Electrical Response of YBa.sub.2 Cu.sub.3 O.sub.7-x ", J. Eur. Ceram Soc. 5, 81-85 (1989).
It has further been established that the nature and the chemical composition of the grain interfaces are crucial to achieving intergrain coupling, as indicated in the following articles in particular:
"Superconducting Transport Properties of Grain Boundaries in YBa.sub.2 Cu.sub.3 O.sub.7 ", Phys. Rev. B 41, 4038 (1990), PA0 "Evidence for local composition variations within YBa.sub.2 Cu.sub.3 O.sub.7-x Grain Boundaries", Appl. Phys. Lett. 55(4) 393 (1989). PA0 The first is that the main source of mechanical stresses in YBaCuO ceramics is the anisotropic coefficient of thermal expansion. Differential shrinkage on cooling causes stresses that can exceed the rupture strength and contribute to microcracking. This problem could be remedied by using particles of very regular dimensions (plates or disks, for example) stacked according to their morphology. PA0 The second is that the use of microcrystalline YBaCuO disks is a priori favorable to obtaining oriented or even textured ceramics. PA0 starting with a powder of composition: PA0 the powder is compressed and sintered at a temperature below 920.degree. C. (the BaCuO.sub.2 and CuO binary eutectic temperature) to form a sample, PA0 said sample is placed on an oxide of said lanthanide R, PA0 the sample and its support undergo heat treatment enabling chemical reaction between the liquid part of said sample and its support whereby substantially all of said liquid part is consumed and highly regular 123 monocrystals are obtained, PA0 cooling is applied, PA0 at least one annealing is carried out in pure oxygen at a temperature between 350.degree. C. and 500.degree. C. to obtain the orthorhombic form characteristic of RBa.sub.2 Cu.sub.3 O.sub.y. PA0 An increase in the initial particle size through the action of a liquid phase which yields parallelepiped-shape plates. PA0 Extraction of the liquid by wettability on the lanthanide oxide support. PA0 two lanthanide oxide support plates are provided for said sample, PA0 said sample is disposed between said supports, during said heat treatment, a pressure in the order of at least ten Pascals is applied between said supports, said microcrystals being then oriented perpendicularly to the direction of the pressure force.
Two important conclusions can be drawn from the current state of the art.
The major technical problem to be solved is that of obtaining microcrystal powders with the required particle size properties.
K. DEMBINSKI, M. GERVAIS, P. ODIER and J. P. COUTURES in the publication "A non Polluting Single Crystal Growth Process for YBaCuO and Phase Diagram related to", E-MRS 90, March 1990, have given the essential characteristics of the Y.sub.2 O.sub.3 --BaO--CuO phase diagram and in particular those of the 123/7BaO--18CuO section. According to this article, compositions between A [0.08(123)--0.92 (7BaO--18CuO ] and B [0.70(123)--0.3 (7BaO--18CuO ] are heated to temperatures between 930.degree. C. and 980.degree. C.
The 123 phase is formed in the presence of a liquid. The state of the art in respect of other materials suggests that there should occur in this liquid a "disolution-reprecipitation" phenomenon whose result should be the formation of microcrystals of the 123 phase trapped in a matrix resulting from the crystallized flux.
An object of the present invention is to propose a method of obtaining these microcrystals powders by flux extraction, this method being adapted to yield well oriented microcrystals.